Cholecalciferol
is a form of vitamin D, also called vitamin
D3 or calciol.[1][2]

It
is structurally similar to steroids such as
testosterone, cholesterol, and cortisol (though
vitamin D3 itself is a secosteroid).
Cholecalciferol has a molecular weight of 384.64
g/mol and molecular formula of C27H44O.

Vitamin
D3 has several forms:

Cholecalciferol,
(sometimes called calciol) is an inactive,
unhydroxylated form of vitamin D3)

Calcifediol
(also called calcidiol, hydroxycholecalciferol,
25-hydroxyvitamin D3, etc. and
abbreviated 25(OH)D is one of the forms
measured in the blood to assess vitamin
D status[3]

Calcitriol
(also called 1,25-dihydroxyvitamin D3)
is the active form of D3.

Metabolism
of Vitamin D3

7-Dehydrocholesterol
is the precursor of vitamin D3 and
forms cholecalciferol only after isomerization
due to UV radiation exposure from the sun or
artificial sources of UVB light.

Cholecalciferol
is then hydroxylated in the liver to become
calcifediol (25-hydroxyvitamin D3).

Next,
calcifediol is again hydroxylated, this time
in the kidney, and becomes calcitriol
(1,25-dihydroxyvitamin D3). Calcitriol
is the most active hormone form of vitamin D3.

Dose

One
gram of pure vitamin D3 is 40 000
000 (40x106) IU. Recommendations
are: 15 micrograms (600 IU or International
Units) daily for all individuals (males, female,
pregnant/lactating women) under the age of 70
years-old. For all individuals older than 70
years, 20 micrograms daily (800 IU) is recommended.[4]

A
growing body of researchers question whether
the current recommended adequate levels are
sufficient to meet physiological needs, particularly
for individuals deprived of regular sun exposure
or those at higher risk such as those with higher-melanin
content in the skin (i.e. those whose ancestors
are African, Latin American, or Asian), the
obese, and those who live far from the equator.

The
upper limit (UL) for vitamin D has been recommended
as 4,000 IU daily. The 4,000 IU cut-off was
determined by the Institute of Medicine in 2010
after reviewing the then-current medical literature,
finding that toxicity had consistently occurred
when doses of 40,000 IU daily were taken,[5]
and that there was a single case of toxicity
above 10,000 IU daily; this case of toxicity
occurred under circumstances which have led
other researchers to dispute it as a credible
case to consider when making vitamin D intake
recommendations.[5]
The Institute of Medicine did not find evidence
of toxicity between 4,000 IU and 10,000 IU,
so the 4,000 IU number is more of an estimate
than a number based on evidence of toxicity
above 4,000 IU.[4] Patients with severe vitamin
D deficiency will require treatment with a loading
dose, its magnitude can be calculated based
on the actual serum 25-hydroxy-vitamin D level
and body weight.[6]

Vitamin D toxicity can result from regular excess intake
of this vitamin, and may lead to hypercalcemia
and excess bone loss.

The
25-hydroxy vitamin D (calcifediol) blood test
is used to determine how much vitamin D is in
the body. The normal range of calcifediol is
30.0 to 74.0 ng/mL.[3]